We propose to examine by Cu and Fe EXAFS the cytochrome bo quinol oxidase of E. coli as an analog of the mammalian cytochrome c oxidase ( aa3). Cytochrome bo has been cloned and expressed and site-directed mutated proteins have been characterized. Since cytochrome bo is one member of the cytochrome oxidase superfamily that does not contain the Cu A site, Cu EXAFS will be able to directly probe the Cu B coordination environment without interference from the other Cu site. We will take advantage of this characteristic to explore the structure of the dinuclear heme o-CuB site in various redox states, and in the presence of substrate O2, competitive inhibitors, and exogenous ligands.